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Vintage Amateur and Military RadioAmateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

Can any anyone ID the (possibly military) gizmo in the attached photos? It is mostly brass with an ebonite or similar top and the base has four non-brass solid pins in a British B4 pattern, all drilled into the base. The insulated top has a single terminal and the base and top are riveted together.

The top is marked "X1141" and "958" is also scratched onto the top, possibly a serial number. The "anode" pin of the B4 is marked as "5" and finally the middle of the underside is drilled for a 4BA screw.

Plugging this into a B4 base would obviously short all the pins together, so a piece of test gear comes to mind. There is no electrical connection between the terminal and the base and pins, measuring on the highest range of my mulitmeter and there is no discernable meter "kick" which might mean there is a capacitor inside.

I'm betting that it's either a crystal or some sort of plug-in capacitor: I recall that WWII-era transmitters sometimes used plug-in "antenna-shortening" capacitors (in series with the antenna) to help a radio designed for short whip-antennas (well less than 1/4-wave) to match a longwire that may be more than 1/4-wave long.

I'm betting that it's either a crystal or some sort of plug-in capacitor: I recall that WWII-era transmitters sometimes used plug-in "antenna-shortening" capacitors (in series with the antenna) to help a radio designed for short whip-antennas (well less than 1/4-wave) to match a longwire that may be more than 1/4-wave long.

No, the brass fittings on the top appear to hold the ebonite(?) terminal board to the brass bottom, I think they are rivets as they are slightly proud of the board, suggesting a flat head?

The pins in the base definitely fit snugly into a B4 base, if by B4 you mean a PM2 valve base. It's not the American 4-pin base which has large and small pins. The base pins do not align with the (?) rivets on top, which are at 90 degree points around the board.

If I get chance tomorrow, I will hook it up to my sig gen which has a crystal tester built-in and I will also try it on my capacitance bridge. If it does have a antenna shortening function then I would expect the capacitor to be of a low value, 100pf at most.

OK, it is a crystal! Oscillates at 2017.88 kHz on my tester. I can't see that the X1141 and 958 numbers on the top have any relation to the frequency it works at.

Presumably, there was a B4 crystal holder or holders in the equipment and selection was made by a flying lead. The freq may indicate marine band but also older military kit, something like the early numbers of the Wireless Sets range?

Yes, Lawrence, a good guess! I've had a quick look thru Wireless for the Warrior Compendium 1, which covers the Spark to Larkspur era and I cannot see anything that resembles the crystal. Most of the post-WW1 sets had master oscillators and only the later WS No 1 and 3 onwards had crystal oscillators as an option.

If it is marine band, then a pre-WW2 list of ship working frequencies might show the crystal as being for a ship TX. I have some data on that subject, if I can find it!

Crystals have two resonance modes, series and parallel, and it could have been used on either. So does either mode land on a popular maritime frequency?

I have a copy of a report on the RMS Queen Mary's radio installation dated 1936 and that shows LF and MF freqs from 111.5 up to 500kHz. After that, the next freq they list is 8354kHz. It looks likely that there wasn't an allocation to the Maritime Mobile Service around 2MHz pre-war, if there was it's odd that a high-traffic ship like the QM didn't make use of it.

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